You probably don’t need this. When you buy an Athena kit, you receive a Bridge Board as part of the kit.
The Bridge Board provides power to the BeagleBone and allows it to communicate with and upload firmware to the Melzi. This is the only part of the Athena that can neither be printed, nor bought from regular stores. If you source the parts for your Athena yourself, or need a replacement, you can buy this part. Alternatively, you can power the BeagleBone through its USB port and use the schematic of the Bridge Board to create the required connections.